Thanks Juzaam. Actually, though, I can't take credit for the deep analysis! I posted the same question on the Role Scoper forum, and the author, Kevin, kindly did the bug-hunting. Apparently, some other plugins make the same file call, only in those it seems easier to fix. (In truth, I have no idea what either of you programmers are talking about!)

In any case, Kevin is willing to collaborate if you are interested in fixing this problem so that your two plugins can live together in peace and harmony. :-)

Here's his reply on the Role Scoper Forum:
"I think you should talk it over with the DRegister author (Ezio Lanza - Juzaam). I would be happy to confer with him if any potential and interest is there."

And his website: <a href="http://agapetry.net/plugins/" target="_blank">http://agapetry.net/plugins/

Do let me know if you two fix the problem, as I would very much like to use your plugin.

Thanks!